fix[lang]: fix incorrect handling of tuples in nested subscripts

[Sporarum]

What I did

Fix #4728

How I did it

How to verify it

Commit message

this commits adds some logic to handle typechecking of nested subscripts
in the presence of tuple types. it assumes the only valid indices for
tuples are constant integers, this is guaranteed by the parser.

it also adds a new test that checks these kinds of nested subscripts.

Description for the changelog

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.30%. Comparing base (31122dc) to head (df4b8d5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4776   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files         137      137           
  Lines       19465    19475   +10     
  Branches     3364     3366    +2     
=======================================
+ Hits        18161    18171   +10     
  Misses        884      884           
  Partials      420      420           

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